Immunosuppressant drugs such as tacrolimus, everolimus, temsorolimus and cyclosporine are effective for the treatment of organ or tissue rejection following transplant surgery, of graft versus host disease and of autoimmune diseases in humans. During immunosuppressant drug therapy, monitoring the blood concentration levels of the immunosuppressant is an important aspect of clinical care because insufficient drug levels lead to graft (organ or tissue) rejection and excessive levels lead to undesired side effects and toxicities. Blood levels of immunosuppressant are therefore measured so drug dosages can be adjusted to maintain the drug level at the appropriate concentration. Diagnostic assays for determination of immunosuppressant blood levels have thus found wide clinical use.
A commonly used competitive format involves the binding of a first antibody to the immunosuppressant and the binding of labeled immunosuppressant (e.g., acridinium-tacrolimus) to the remaining free antibody binding sites, followed by quantitation by detection of the label.